1. Field of the Invention
The present invention relates in general to solid-state image sensing devices, and in more particular to solid-state imagers for use in television cameras of higher resolution. More specifically, the invention relates to charge coupled device (CCD) image sensors preferably employed in highly advanced television image pickup systems, including a simultaneous all-pixel read system (called the "progressive scanning" system), an enhanced vertical definition system known as "EVS," or the like.
2. Description of the Related Art
With the increasing needs for high performance of television broadcasting systems, development of a highly advanced solid state image sensing device with an extra high resolution and high reliability has been demanded strongly. Even the presently available charge-coupled device (CCD) image sensors have accomplished several hundreds of thousands of picture elements (pixels), which are increased in number enough to meet the requirements in the existing television broadcasting systems, such as NTSC, PAL, or the like. However, by taking account of the possible application to a new high-definition television broadcasting system known as "HDTV" in the near future, CCD imager sensors will soon be demanded to further increase in the pixel density, thus providing an extremely greater number of pixels that ranges from one million and three hundreds of thousands to two millions.
A problem raised when the CCD image sensors are forced to attain such greater number of pixels is an increase in the frequency of drive pulse signals thereof. As the drive frequency increases, the risk of delaying the drive pulses and/or "rounding" the drive pulse waveform may increase due to the presence of the resistance and the parasitic capacitance of the transfer electrodes of the CCD image sensors. This may affect seriously the image pickup performance of such CCD imagers.
One advance made in the prior art to overcome such problem is the multilayered charge transfer electrode structure, wherein a low-resistance metal wiring layer is stacked on each of the transfer electrodes (typically, the vertical charge transfer electrodes), causing the resistance thereof to reduce as a whole to thereby suppress or prevent the occurrence of pulse delay and the rounding of pulse waveform. Such extra low-resistance wiring layer is well-known as the "backplate wire" or "shunt electrode" among experts in the art of CCD devices. The shunt electrode is electrically connected to a corresponding one of the charge transfer electrodes by means of a plurality of contact holes.
A CCD imager having the stacked transfer-gate/shunt electrode structure is disclosed, for example, in "Frame Interline Transfer CCD Sensor for HDTV Camera," by T. Nobusada et al., 1989 IEEE International Solid-State Circuits Conference (ISSCC), 1989 at p. 89 (FIG. 1). With the "Poly-Si/Al double-layer" transfer gate structure disclosed therein, a plurality of parallel insulated aluminum wires extend on vertical charge transfer gates above a CCD substrate. Contact holes are arranged at the preselected positions between the aluminum wires and the vertical charge transfer gates to provide electrical interconnection therebetween. As is apparent from viewing FIG. 1 of the IEEE paper, the contact holes are specifically distributed so that these contacts are positioned in a diagonally extending direction to the elongate direction of the parallel aluminum wires (shunt electrodes).
Unfortunately, the advantages provided by the CCD imager do not come without accompanying penalties. Since the contact holes for the aluminum wires are formed directly on the underlying vertical transfer gate layers, the charge transfer channel regions formed in the surface of the substrate may vary in potential at specific portions beneath the contacts of the transfer gate layers. Such undesirable potential variation in the charge transfer channel regions leads to the occurrence of a partial potential shift therein, as has been known as the "potential pocket" among those skilled in the CCD device art. The occurrence of potential pocket is a bar to the achievement of smooth flow (or vertical transfer) of charge packets indicative of a sensed image in the charge transfer channels. The charge transfer rate is thus reduced causing the CCD imager to decrease in the image pickup performance.
Another CCD imager with the stacked transfer-gate/shunt electrode structure is disclosed, for example, in "A 2 Million Pixel FIT-CCD Image Sensor for HDTV Camera System," by K.Yonemoto et al., 1990 IEEE ISSCC, 1990 at p. 215 (FIG. 2). With the FIT-CCD imager, a vertical charge transfer electrode made from polycrystalline silicon (poly-Si) is provided with a stacked shunt-layer structure consisting of a poly-Si layer and an aluminum wiring layer stacked on each other. The poly-Si layer extends above the vertical charge transfer electrode, whereas the aluminum wire runs above the poly-Si layer to provide a triple-layered charge transfer gate electrode structure. The intermediate poly-Si layer between the underlying transfer electrode and the overlying aluminum layer is called the "buffer electrode" in some cases. Another saying of this is that the buffer electrode is additionally inserted between the transfer electrode and the shunt electrode. The presence of such intermediate buffer electrode can successfully suppress the occurrence of a potential pocket in the transfer channels in the substrate.
With the multiple-layered lamination structure for the charge transfer gate electrode, the electrical coupling between the transfer electrode and the poly-Si buffer electrode is achieved by a plurality of first contact holes formed therebetween. The electrical interconnection between the buffer electrode and the aluminum shunt electrode is attained by the use of a plurality of second contact holes arranged between them. The first and second contact holes are specifically distributed on the parallel transfer gate electrodes so that the first and second contact holes do not occupy the same planar positions.
More specifically, the distribution pattern of the second contact holes (or, "buffer-to-shunt" contacts) is defined with four pixels as a unit in the horizontal direction on the substrate surface of the CCD imager. The repeat period is thus four pixels. This means that, looking at a matrix of rows and columns of pixels as a whole, the distribution or positioning pattern of the first and second contacts around the photoelectrically converted charge storage sections of such pixels does not remain uniform. For example, around the charge storage section of a certain pixel, two "first" contact holes and one "second" contact hole are located; on the other hand, only one second contact hole is merely formed around the charge storage section of another pixel. As a result of the use of such layout pattern of first and second contact holes, four different kinds of patterns are present due to the fact that the repeat period is four pixels. The coexistence of such different kinds of first/second contact-hole patterns inevitably causes the incident-light entrance characteristic to be unable to remain uniform; this results in that the sensitivity locally varies among an increased number of pixels on the substrate.
A more significant problem faced with the prior-art CCD imager is the fact that the coexistence of four kinds of contact patterns, which is resulted from the arrangement that the repeat period of the second contact holes ("buffer-to-shunt" contacts) in the horizontal direction X is defined with four pixels being as a unit, may accelerate noises to occur so that the resultant signal-to-noise ratio decreases, for the reasons as follows. As the repeat period of buffer-to-shunt contacts is four pixels, the spatial frequency fc of the contact pattern in this case is a quarter of the CCD sampling frequency fs. On the other hand, as is well known, the upper limit of the frequency band of an image signal is slightly lower than the half of the sampling frequency fs. Naturally, the spatial frequency fc of the contact pattern falls within the image-signal frequency band. This may cause noises to take place in the image signal.